High frequency boost converters have been widely used in various power conversion applications such as single phase and three phase power factor corrected AC/DC switch-mode rectifiers. The boost stage processes the AC input and develops a DC voltage that is typically between 400 volts and 800 volts depending on the input voltage level. Boost converters are usually the topology of choice for providing a high output voltage from substantially lower DC voltages. In applications with universal AC input voltages (e.g., 85 to 264 VAC) where an intermediate DC bus voltage of 400 volts is necessary, a soft-switched boost is often used to obtain a boost converter efficiency of 96% or better.
Major problem areas with high frequency, high power boost converters revolve around a reverse recovery current condition associated with the boost diode(s) during a turn-on transition of a main boost switch, and switching losses associated with the main boost switch of the converter. The switching devices of the boost converter are generally operated at very high frequencies thereby allowing the use of small energy storage elements and filtering components (such as inductors and capacitors) in the converter. As the switching frequency of the switching devices is pushed even higher to increase the converter power density, the reverse recovery condition associated with the boost diode(s) and the turn-on and turn-off losses associated with the main boost switch become more severe. A significant reverse recovery current may at worst damage or destroy both the boost diode(s) and the main boost switch and at best contribute to poor power conversion efficiency.
Other problems arise when high blocking voltage rated switching devices are required. The cost of the high blocking voltage rated switching devices is much higher than the lower voltage rated switching devices. Additionally, the higher voltage rated devices exhibit higher forward conduction voltage drops than the lower voltage rated devices which makes them more lossy and therefore less efficient.
To deal with these problems, various passive and active snubber circuits have been developed to address and compensate for these undesirable qualities. Some of these snubber circuits are very complicated and difficult to implement. Many have high losses and therefore contribute to lower converter efficiency which, while offering protection to the boost diodes and switches, just transfers the overall power loss to the snubber circuit.
Among the snubber circuits developed, the energy recovery snubber circuit with reduced turn-off loss is one of the more attractive operationally. The energy recovery snubber circuit, however, requires six to eight additional circuit components which often makes the circuit layout challenging in terms of minimizing stray inductance. Stray inductance causes spurious "ringing" at switching transition times which often significantly increases the voltage stresses on boost devices if left uncompensated. Furthermore, limiting the diode reverse recovery current too severely will not allow the circuit to function properly.
Another snubber circuit design is the simple resonant, nondissipative turn-off snubber circuit. This snubber circuit was developed for transformer isolated flyback and forward converters to protect the main switch from excessive voltage stress produced from the energy stored in the leakage inductance of a power transformer during the turn-off transition of the main switch. However, the resonant snubber itself may still produce larger than desired voltage stresses across the switch(es) and diode(s) when used for the boost converter.
Accordingly, what is needed in the art is a circuit that limits the voltage stresses on the critical devices of a boost converter and overcomes the deficiencies in the prior art.